The present invention relates to equivalent electric dipole localization systems and, more particularly, to an equivalent electric dipole localization system, in which the nerve activities of a living body are represented by an electric dipole for obtaining information about the electric dipole sources and also information about the confidence region or confidence limit of the estimated electric dipole inversely from potential distributions projected on the organic surface. The system localizes equivalent current dipole in living tissue.
Heretofore, electroencephalographs, electromyographs, evoked potential adders, etc. have been used to measure potentials evoked on the organic surface by nerve activities of living bodies. Also, recently an equivalent dipole localization method has been proposed, in which activated areas in the living body are estimated by measuring potentials evoked on the organic surface by the nerve activities of the living body. In this method, with stimulation of brain cells in the activated areas, electromotive forces are generated to create potential distributions on the scalp. From such potential distributions, the individual activated areas are corresponded by an electric dipole, and the location and vector component of the dipole are calculated from the potential distributions noted above to estimate the locations of the active brain cells, thereby obtaining numerical representation of the active status of the brain. In a specific proposed method of calculation, a plurality of electrodes are fitted on the organic surface, and potentials generated on the electrodes by the nerve activities of the living body are measured simultaneously. Then, an electric dipole is assumed in a predetermined location in the living body has a medium having a certain character, and the potentials at the locations of the electrodes created by the electric dipole are calculated. Further, the square error between the measured and calculated values obtained for each electrode is obtained, and the location and vector component of the electric dipole that minimize the square error are obtained as those of an equivalent electric dipole.
In the above prior art equivalent electric dipole localization method, the error function to be minimized is a least square error function of the measured and calculated potentials obtained with each electrode. Such an error function, however, is effective only when there is no background noise correlation among the individual electrodes. For example, S. Kuriki, M. Murase and F. Takeuchi, "Locating accuracy of a current source of neuromagnetic response: simulation study for a single current in a spherical conductor", Electroencephalography and Clinical Neurophysiology, 1989, Vol. 73, pp. 499-506, report that errors contained in dipole locations estimated to be greatly electroencephalographically affected amount to 1 cm. Brain waves evoked by visual stimulation may contain, in one waveform, components with close cortical regions such as regions 17 and 18 as sources, and it is very difficult to separate such components and specify the locations thereof with a dipole expressing the individual sources.
What are known about the spacial characters of the background brain waves are surprisingly few, but there are known characters in the cerebrum, and similar characters could be estimated about signals measured on the scalp. For example, J. C. De Munck, P. C. M. Vijn and H. Spekreijise, "Random dipoles as a model for spontaneous EEG- and MEG activity", Advances in Biomagnetism (edited by S. J. Williamson et al., Plenum Press, New York, 1989), show that the spacial characters of background brain waves can be described in terms of a linear relation between variance and inter-electrode distance. If such a spacial correlation is utilized, it will be possible to select better functions as the error function noted above, and the conventional least square error function may be utilized after removing such spacial correlation by filtering.